1. Field of the Invention
The present invention relates to methods of inspection usable, for example, in the manufacture of devices by lithographic techniques
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is necessary to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
Devices are built up layer by layer and overlay is a measure of a lithographic apparatus' ability to print these layers accurately on top of each other. Successive layers or multiple processes on the same layer must be accurately aligned to the previous layer, otherwise electrical contact between structures will be poor and the resulting devices will not perform to specification. Overlay is a measure of the accuracy of this alignment. Good overlay improves device yield and enables smaller product patterns to be printed. The overlay error between successive layers formed in or on the patterned substrate is controlled by various parts of the exposure apparatus (of the lithographic apparatus). It is mostly the alignment system of the lithographic apparatus that is responsible for the alignment of the radiation onto the correct portions of the substrate.
Overlay may be measured using an “image-based” (box-in-box) technique or Diffraction-Based Overlay (DBO) metrology. DBO is an emerging metrology technique used because of its superb TMU (Total Measurement Uncertainty) compared to “image-based” techniques. In the “image-based” case, overlay may be derived from a measurement of the position of a resist marker pattern relative to a marker pattern in an earlier formed product layer. In the DBO case, overlay is indirectly measured, for example by detecting variations in diffracted intensities of two overlapping periodic structures such as a top resist grating stacked over a product layer grating.
Diffraction based overlay (DBO) usually measures differences in intensity between positive and negative diffraction orders (asymmetry) obtained from a radiation source beamed upon a grating or similar structure. The grating is made up of at least two overlaid layers, and the resulting diffraction orders should be symmetrical if there is no overlay offset between the two layers. Where there is asymmetry, the coherence between these asymmetries and overlay numbers is typically a recurrent function with unknown shape depending on stack and the illumination conditions. For small overlay values, this shape can be approximated by approximately linear region of a first order sine curve. Consequently, for such small overlay values, the asymmetry can be assumed to be proportional to the overlay: A=K×OV. However, for overlay errors greater than 15 nm this fit is not valid anymore and leads to significant measurement errors